A conventional image reading device includes a conveyor path, an illuminating device, and a camera. When a sheet is conveyed by the conveyor path arrives at a reading position, reflected light from the sheet which is illuminated by the illuminating device is condensed to a line sensor by a lens of the camera. The line sensor is scanned in a direction orthogonal to a conveying direction, so that the reflected light from the sheet is read in synchronization with the scanning. Data thus read is converted into a two-dimensional image by an image acquisition portion.
However, the image reading device thus configured has such a problem that the conveyance of sheets causes the sheets to be uneven in a height direction of the sheets and light intensity distribution gradients (distribution) in the height direction are obtained as brightness of the sheets without any change when image reading is performed. In order to solve this problem, there is suggested a method to equalize the light intensity distribution in the illuminating device in the height direction.
For example, Japanese Patent Application Laid-Open No. 2008-219244 discloses a reading device in which illuminating means formed in combination of an illumination and a rod-like light guide includes a first emission portion for illuminating a sheet obliquely, and a second emission portion for emitting light horizontally in conveying direction, and which is arranged to reflect light from the second emission portion and to obliquely illuminate the sheet in an opposite manner to light from the first emission portion with respect to an optical axis of image reading of the sheet, thereby resulting in that the reading device has resistance to flutters and tilts of sheets.
Further, Japanese Patent Application Laid-Open No. 2010-483163 discloses a reading device in which a cylindrical optics is disposed in front of a rod-like illumination, and the illumination, which emits quite parallel light, obliquely illuminates a sheet from two directions opposite to each other with respect to an optical axis of image reading of the sheet, thereby resulting in that the reading device has resistance to flutters and tilts of sheets.
However, in these conventional image reading devices described above, the quantity of reflected light (reflecting luminance) from a sheet varies depending on a distance between a white reference plate and the sheet. That is, when a sheet flutters on a top surface of the white reference plate disposed at a reading position, the distance between the white reference plate and the sheet varies. This varies the quantity of reflected light from the sheet, thereby resulting in that, as an image reading device, the conventional image reading devices cannot obtain an image suitable for human sight. Further, as a sheet inspection device, the conventional image reading devices cannot obtain a stable reflection image, thereby causing such a problem that highly precise inspection cannot be performed. Particularly, a degree of influence by the white reference plate is different between a case of a thick sheet and a case of a thin sheet, thereby causing such a problem that these sheets have different quantities of reflected light from their surfaces.